Recovery of aromatics from watercontaining cracked gas mixtures



NOV. 15, 1960 T. J. DUPY 2,960,547

RECOVERY oF ARoMATIcs FROM WATER-CONTAINING CRACKED @As MIxTuREs Filed July 29, 1957 United States Patent `RECOVERY F` ARQMA'HCS FRM 'WATER- )CDNTAINING CRACKED GAS MIXTURES 1Thurman."J..Dupy, Port Arthur, Tex., assignor to Gulf .Oil forporation, Pittsburgh, kPa., a corporation of i-Pennsylvania l `ll'iled July"29, 1957, SenNo. 674,890

J6 Claims. (Cl. 260-674) inventionrelates to a process for recovering organic material andrparticularly to a process for recover- `ing yaromatics from a cracked `gas mixture.

It yis customary to vsubject a gas Vor gaseous mixture, :s uch as ethane, propane and/or butane to cracking `conditions in order to obtaina mixture containing gases such ,as ethylene, propylene, etc. These .gases .are retcovered from these mixtures and employed .in many chem- @ical -reactions `Included in thecracked gas mixture are aromatics -which would also form a desirable product from the cracking reaction. The recovery of aromatics fromsuch rmixture is exceedingly diilicult, however. In itiallyrX the cracked `gas mixture obtained-is Vcooled from V,a temperature of about l400 to about 1550 F. to a temperature of about A50 to about 400 by' contacting the same with V,cooling vwater. The cracked gases -are .easily recovered, thereafter, `but the water used in cooling the gases combines with the aromatics present and forms an `emulsion therewith. Separation of fthe aromatics from the resulting emulsion vis an exceedingly `f lilcult problem, and as a result thereof much of the aromatics Vobtained in the cracking reactionislost I have found that the recovery of aromatics from :such emulsions can be effected by the expedient ,of adding an alkaline agent to such emulsion to break'thegsame .and obtain a separation of water and aromatics. The water and aromatics can then be separately recovered.

,Azpreferred `method of` carrying outthe invention can `be illustratedtby reference to the Vsingle drawing which `describes-a flow diagram of such preferred procedure. vvvI'lle ,drawing ishereby incorporated in and made apart of this specification. .In order to simplify the description ofthe process, ow devices such as valves, etc., have not been shown.

The ,gases to be cracked, ethane, propane, and/or butane, for example, are `introduced into the system by line 2, preferably at a temperature of `about 100 to 300 F. Prior Ato introducing the gases into cracking heater 4, they are Vjoined by steamin line 6 for the pur- .pose of controlling the space velocity of the gases .through cracking heater 4 and I-to inhibit ,.coking. The amount of steam which can be used `can be about l0 to about 100 percent by volume based on the gases introduced in line 2. The gases are then `passed through cracking heater 4 at a space velocity .of about 4,000 to about 14,000 pounds per square foot per hour for about 0.005 to about 0.025 minute. A temperature of about l400 to about 1550 F. is maintained in cracking heater4 by -means not shown, and the pressure can be from about atmospheric to about 50 pounds per square inch gauge.

As a result of the cracking conditions maintained within-cracking heater 4, a cracked gas mixture is obtained containing C1 to C6 hydrocarbons, saturated as well vas unsaturated, water or steam, hydrogen, carbon dioxide and aromatics comprising benzene, toluene, xylene and tats. The aromatics comprise about 0.1 to about .one percent by volume of the cracked gas mixture.

*The cracked gas mixture described above isremoved 'Lice from cracking heater .4 by line 8 and introduced .into quench tower 10 wherein it is cooled by cooling water, in the form of a spray, which is introduced therein. by one or `more lines .12. The cooling water can be at a temperature of about 40 to about 250 F. ,About 10 vto about 30 parts of water per part of gas is suicient to effect the desired cooling. In ,order to inhibit .secondary reactions from arising within .the cracked .gas mixture, it is important that the cooling be etectedin less than about one second.

Under the conditions existing in the quench-tower, the hydrocarbons, hydrogen and carbon monoxide are-cooled to a temperature of about 50 to about 300 F. andremoved overhead by line 14 for subsequent use. The cooling water and aromatics remaining in quench tower r1.0 Aare found in the form of an emulsion and are removed therefrom by line 16 at a ,temperature of about 100.to about 400 F. 'I'he emulsionhas a pH of about 2to about 6.8, generally about 3 to 4. The aromatics present constitute about 0.01 to about 10 percent by weight of the emulsion.

The aromatics present in the emulsion removed from quench tower are valuable commercially. However, great diiliculty has been experienced in recovering -sucharomatics from the emulsion. In accordance with -theproc- 4ess of this invention, an alkaline agent whichwill -not adversely alect the aromatics present is added rthereto in an amount suicient to raise the pH of such emulsion to about 7.1 to about l2, preferablyabout 9 to 1.0. This is eifective in breaking the emulsion and obtaining a separation of water and aromatics. The amount of ;.alkaline agent needed is that required to obtain a ,pH inthe resultant solution `as described above. Generally about 0.05 to about 5.0, preferably about 0.1 to about one,per cent .by weight ofalkaline agent relative to the emulsion is effective for such purpose. Temperature .ofttreatment is not critical and can be from about .to :about 400 F. As alkaline agent any such agent can be used which will produce the desired pH in theresultant ture without adversely atfecting the aromatics present. Thussuch alkaline agents as sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, ammonia, calcium carbonate, sodium carbonate, sodium bi.- carbonate, potassium bicarbonate, etc. can eifectivelybe used.

In accordance with the above, therefore, the ,emulsion is introduced into water settler and separator 18 by line 16 and the alkaline `agent by line 20. The alkaline agent and emulsion can be mixed in water settler and separator 1S by any convenient means. A contact time of about l minute to about 1l minutes is sufficient tot obtainthe desired pH and resultant separation. Upon breakingof the emulsion, it straties into an upper aromatic vlayer and a lower aqueous layer. The latter is removed from water settler and separator 18 by line 22 and the aromatic layer by line 24. The water removed by line 22 canbe recycled to constitute the cooling water used in line 12. The composition of the aromatic fraction removed by line 24 can have the following speciiication:

Percent Vby weight Olens About 2 to about 'l0 Benzene About 25 to about`35 Toluene About 15 to about 25 Xylene About 2 to about '10 Tars About 2 to about ll0 Water Aboutl to-abou-t 5 Further separation lof the aromatic fraction in to its V individual components can be eifected in any desired 1or. oo n-4 ventional manner.

The invention can further be illustrated by reference to the following examples. f

Example I 22,000 pounds of ethane, 64,000 pounds of propane and 16,000 pounds of butane per hour were introduced along with 42,000 pounds of steam at 240 pounds per square inch gauge and 500 F. into a cracking heater and cracked at temperatures of 1475 to 1520 F. and 10 pounds per square inch gauge to obtain 102,000 pounds of gases comprising C1 to C6 saturated and unsaturated hydrocarbons, carbon dioxide, hydrogen and 0.3 percent by volume of aromatics comprising benzene, toluene and tars. The cracked gas mixture was passed to a quench tower where it was contacted with 4290 gallons of water having a temperature of 85 F. The gases were cooled to a temperature of 95 F. and removed overhead from the quench tower. The water and aromatics formed an emulsion having -a temperature of 175 F. The aromatics formed 0.047 percent by weight of the emulsion. The emulsion having a pH of 3.5, was passed to a settler and allowed to stand for 11 minutes. There was no indication that a separation of water and aromatics had or would take place. Similar results were obtained with other emulsions of the same character having pHs of 3.5, 3.8, 4.0 and 4.3.

Example Il A run similar to the above was made except that 0.003 percent by weight of a solution containing 5 percent by weight of sodium carbonate and 3 percent by weight of sodium hydroxide was added to the emulsion and the resulting solution, having Va pH of 8.0, was permitted to stand 11 minutes. A clean separation of aromatics and water was obtained. Similar satisfactory yresults were obtained when the same alkaline solution was added to the emulsion to obtain resulting mixtures having pHs of 7.2, 8.4, 9.1, 9.2, 9.8 and 10.0.

Obviously, many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim: j

1. In a process for recovering aromatics from a cracked gas mixture containing the same wherein a hydrocarbon l is subjected to cracking conditions to obtain a cracked gas mixture containing gases and aromatics, the cracked gas mixture is cooled by direct contact with water, the gases are separated from said cooled cracked gas mixture, the pH of said cooled cracked gas mixture being from about 2 to about 6.8, and aromatics are separated from the resulting cooled cracked gas mixture, the improvement which comprises adding an alkaline agent to said cracked gas mixture to obtain a resulting mixture having a pH of about 7.1 to about 12, whereby the aromatics present in said cooled cracked gas mixture which would not have formed a separate phase in the absence of such addition are thus formed in a separate phase and the recovery thereof thereby facilitated.

2. In a process for recovering aromatics from a cracked gas mixture containing the same wherein a hydro-carbon is subjected to cracking conditions to obtain a cracked gas mixture containing gases and aromatics, the cracked gas mixture is cooled by direct contact with water, the gases are separated from said cooled cracked gas mixture, the pH of said cooled cracked gas mixture being from about 2 to about 6.8, and aromatics are separated from said cooled cracked gas mixture, the improvement which comprises adding an alkaline agent comprising sodium carbonate to said cracked gas mixture to obtain a resulting mixture having a pH `of about 7.1 to about l2, whereby the aromatics present in said cooled cracked gas mixture which would not have formed a separate phase in thel -absence of such addition are thus formed in a separate phase and the recovery thereof thereby facilitated.

3. In a process for recovering aromatics from a cracked gas mixture containing the sa'rne wherein a-hydrocarbon is subjected to cracking conditions to obtain a cracked gas mixture containing gases and aromatics, the cracked gas mixture is cooled by direct contact with water, the gases are separated from said cooled cracked gas mixture, the pH of said cooled cracked gas mixture being from about 2 to about 6.8, and aromatics are separated from said cooled cracked gas mixture, the improvement which comprises adding an alkaline agent comprising sodium hydroxide to said cracked gas mixture to obtain a resulting mixture having a pH of yabout 7.1 to about 12, whereby the aromatics present in said cooled cracked gas mixture which would not have formed a separate phase in the absence of such addition are thus formed in a separate phase and the recovery thereof thereby facilitated.

4. In a process for recovering aromatics from a cracked gas mixture containing the same wherein at least one hydrocarbon selected from the group consisting of ethane, propane and butane is subjected to cracking conditions at a temperature of about 1400 to about 1550 F. and a pressure of about atmospheric to about 50 pounds per square inch to obtain a cracked gas mixture comprising C1 to C6 saturated and unsaturated hydrocarbons, Water, carbon dioxide, hydrogen and aromatics, the cracked gas mixture is cooled by direct contact with water in less than about one second to a temperature of about 50 to about 400 F., said C1 to C6 hydrocarbons, carbon dioxide and hydrogen are separated from said cooled cracked gas mixture, the pH of said cooled cracked mixture being from about 2 to about 6.8, and aromatics are separated from the resulting cooled cracked gas/mixture,^the improvement which comprises adding an alkaline agent to said cracked gas mixture to obtain a resulting mixture having a pH of about 7.1 to about 12, whereby the aromatics present in said cooled cracked gas mixture which would not have formed a separate phase in the absence of such addition are thus formed in a separate phase and the recovery thereof thereby facilitated.

5. In a process for recovering aromatics from a cracked gas mixture containing the same wherein at least one hydrocarbon selected from the group consisting of ethane, propane and butane is subjected to cracking conditions at a temperature of about 1400 to about 1550 F. and a pressure of about atmospheric to about 50 pounds per square inch to obtain a cracked gas mixture comprising C1 to C6 saturated and unsaturated hydrocarbons, water, carbon dioxide, hydrogen and aromatics, the cracked gas mixture is cooled by direct contact with Water in less than about one second to a temperature of about 50 to about 400 F., said C1 to C6 hydrocarbons, carbon dioxide and hydrogen are separated from said cooled cracked gas mixture, the pH of said cooled cracked gas mixture being from about 2 to about 6.8, and aromatics are separated from the resulting cooled cracked gas mixture, the improvement which comprises adding an alkaline agent comprising sodium carbonate to said cracked gas mixture to obtain a resulting mixture having a pH of about 7.1 to about 12, whereby the aromatics present in said cooled cracked gas mixture which would not have formed a separate phase in the absence of such addition are thus formed in a separate phase and the recovery thereof thereby facilitated.

6. In a process for recovering aromatics from a cracked gas mixture containing the same wherein at least one hydrocarbon selected from the group consisting of ethane,

propane and butane is subjected to cracking conditions at a temperature of about 1400 to about 1550 F. and a pressure of about atmospheric to about 50 pounds per square inch to obtain a cracked gas mixture comprising C1 to Cs saturated and unsaturated hydrocarbons, water, carbon dioxide, hydrogen and aromatics, the cracked gas mixture is cooled by direct contact with water in less than about one second to a temperature of about 50 to about 400 F., said C1 to C6 hydrocarbons, carbon dioxide and hydrogen are separated from said cooled cracked gas mixture, the pH of said cooled cracked gas References Cited in the le of this patent UNITED STATES PATENTS Averill Feb. -15, Watson Jan. 15, Catanach et al. Sept. 4, Glazier Apr. 3, Krejci June 12, 

1. IN A PROCESS FOR RECOVERING AROMATICS FROM A CRACKED GAS MIXTURE CONTAINING THE SAME WHEREIN A HYDROCARBON IS SUBJECTED TO CRACKING CONDITIONS TO OBTAIN A CRACKED GAS MIXTURE CONTAINING GASES AND AROMATICS, THE CRACKED GAS MIXTURE IS COOLED BY DIRECT CONTACT WITH WATER, THE GASES ARE SEPARATED FROM SAID COOLED CRACKED GAS MIXTURE, THE PH OF SAID COOLED CRACKED GAS MIXTURE BEING FROM ABOUT 2 TO ABOUT 6.8, AND AROMATICS ARE SEPARATED FROM THE RESULTING COOLED CRACKED GAS MIXTURE, THE IMPROVEMENT WHICH COMPRISES ADDING AN ALKALINE AGENT TO SAID CRACKED GAS MIXTURE TO OBTAIN A RESULTING MIXTURE HAVING A PH OF ABOUT 7.1 TO ABOUT 12, WHEREBY THE AROMATICS PRESENT IN SAID COOLED CRACKED GAS MIXTURE WHICH WOULD NOT HAVE FORMED A SEPARATE PHASE IN THE ABSENCE OF SUCH ADDITION ARE THUS FORMED IN A SEPARATE PHASE AND THE RECOVERY THEREOF THEREBY FACILITATED. 